There are known wire sawing devices comprising generally a layer of wires adapted to move with continuous or alternating movement while bearing against a piece to be sawed defining thereby a sawing zone. The sawing zone is constituted of an assembly of cylinders disposed in parallel. These cylinders, called wire guides, are engraved with grooves defining the interval between the threads of the layer, and hence the thickness of the slices. The piece to be sawed is fixed on a support table which moves perpendicularly to the layer of wires. The speed of displacement defines the speed of cutting. The renewal of the wire as well as the control of its tension are carried out in a so-called control portion for the wire located outside the sawing zone properly so called. The agent which will control cutting is either an abrasive fixed on the wire, or else a free abrasive through which the wire runs. The wire acts only as a transporter.
During cutting of thin slices of the piece to be sawed, the stretched wire is both guided and tensioned by the wire guide cylinders. These cylinders generally clad with a layer of synthetic material, are engraved with grooves whose geometry and dimensions must have a high precision. The process of cutting out which uses either a fixed abrasive or a free abrasive, gives rise necessarily to wear of the material which constitutes the surface of the wire guide cylinders, and hence modifies the dimensions and the geometry of said grooves. The wire guide cylinders are conventionally constituted by a hollow or filled monoblock portion clad with a synthetic element into which are engraved the grooves maintaining the spacing of the wires of the layer and fixed at least at one of its ends by one or more blocks including ball-bearings. The wire guide is demountable to be reconditioned after wear of its coating. These wire guide cylinders, which constitute wear elements, have a limited lifetime and the grooves provided at their surface must be periodically reworked by rectification, or turning. This operation takes place away from the sawing device, on a rectifying cylinder or a numerical control tower, for example. The number of reworkings depends on the size of each work, on the geometry of the grooves and of the initial thickness of the cladding. When the cladding is completely used up, one positions on the wire guide cylinder a new coating, by molding, which can be injection molding or any other method which seems suitable for the chosen cladding. When a reworking becomes necessary, the wire guide cylinders must be dismounted and removed from the sawing device. This operation is generally troublesome because of the mechanical complexity of the wire guide cylinders and their drive systems. Moreover, the operation requires the assistance of outside handling. Thus, there is a substantial stopping of the sawing device, which would decrease overall productivity of the assembly. During a change of the thickness of the slices to be sawed, a change of the interval of the grooves out on the surface of the wire guide is necessary and requires also a change of the wire guide cylinders, which is a long and complex and hence undesirable operation.
The precision of the slices, which is very important in electronic applications, depends on the position of the wires in the course of sawing, as well as on the support elements of the piece to be sawed, hence of the position of the wire guide cylinders relative to the piece to be sawed, but also on the quality of the geometry of the grooves cut in the surface of the wire guides. Wear of the grooves gives rise to a degradation of the precision of the slices obtained. It is clear that stopping the sawing device to re-machine the grooves is done only when absolutely necessary and hence there is the tendency to put this off until the last moment, giving rise by this fact to a decrease in the mean quality of the slices thus obtained.
The present invention has for its object to overcome the mentioned drawbacks.
There is thus obtained a very rapid and easy renewal or regeneration of the wire guide grooves and an improvement of the precision of the slices, because the regeneration of the surface is facilitated and uncomplicated. The changes in the interval between the grooves for the modification of the thickness of the slices are also greatly facilitated.
Preferably, the external cladding comprises removable segments adapted to be fixed on the periphery of the cylinders.
This characteristic even more facilitates the operations of renewal and of changing in situ the external portion of the wire guide cylinders.
Thus, to avoid demounting the assembly of the wire guide cylinders for re-machining the grooves, the operation can be carried out by changing only the surface of the wire guide cylinders by giving them a removable surface, by mechanically fixed or cementing, or both. The surface may be solely the synthetic portion, in general of polyurethane, or a series of assembled segments, clad or not with a layer of synthetic polyurethane for example. The machining of the grooves can take place in situ in the sawing device by a re-cutting mechanism which permits re-machining the grooves on the surface of the wire guide cylinders without dismounting the latter, or before mounting the removable segments on a tower or a numerically control alignment device for example. This thus permits regeneration after wear, or the changing of the interval of the grooves for the modification of the thickness of the slices.
This sawing technique, which requires a perfect geometry of the grooves, will enjoy increased precision of the slices by virtue of the fact that the regeneration of the surface is facilitated by the rapid interchangeability of the removable assembled segments and that machining can be done in situ or before mounting, on the removable segments.
The requirements of productivity and of precision of the usages in the semiconductor field, connected to the increasing dimensions of the pieces to be sawed and hence of the slices, require that, even small variations must be avoided. The use of assembled segments, precut or not, permits the regeneration of the surface and the changing of the interval in an easy and hence more frequent manner. Moreover, it requires low supplemental investment because only the segments are necessary and not the whole of the wire guide cylinders.
The invention thus permits the sawing device to use the concept of interchangeability of the surface of the wire guide cylinders and the use of removable segments, to respond to new requirements of technology, by suppressing a long and delicate operation which is the complete dismounting of the wire guide cylinders whilst improving the quality of performance geometrically as well as productivity-wise. The sawing device therefore comprises wire guide cylinders with an interchangeable surface preferably in the form of removable segments held mechanically or by cementing and having their surface precut or not with grooves.
The wire guide cylinders with interchangeable surface will be present in the form of a base cylinder that can receive on its surface and with an indexed position, segments that are coated or not, on which are incised the grooves serving for spacing the windings of sawing wire. These grooves can be pre-cut on an external cutting machine having indexing positions identical to those of the wire guide cylinder, so as to guarantee the position of the grooves after mounting on the wire sawing device.
The use of the concept of interchangeability of the surface therefore permits providing a sawing device performing with high productivity, increased mean precision and permitting easy modification of the thickness of the slices produced.
Other advantages will become apparent from the characteristics set forth in the dependent claims and from the description hereafter set forth the invention in more detail with the aid of drawings which show schematically and by way of example an embodiment and a modification.